1. Field of the Invention
This invention relates to an optical communication method and apparatus used in a field of radio communication employing light, such as infrared light.
2. Description of the Related Art
In the field of radio communication, employing infrared light, there is prescribed the subcarrier frequency allocation by EIAJ (Electronics Industrial Association of Japan) or IEC (International Electrotechnical Commission).
There are a wide variety of equipments for radio communication with infrared light. Most well-known are a so-called remote controller for a television receiver or a video tape recorder employing infrared light and a so-called cordless headphone for receiving music signals transmitted over a radio route from an audio player with infrared light. The subcarrier frequency range used in radio communication with infrared rays in the above-mentioned remote controller is 33 to 40 kHz, while that used in the above-mentioned music signal transmission such as by the cordless headphone is 2 to 6 MHz.
A high-speed communication network, employing infrared rays, and which is made up of a sole control node (equipment) 200 and plural control nodes 260, such as three controlled nodes 260A to 260C, as shown in FIG. 1, is envisioned, and is assumed to do time-divisionally multiplexed communication as shown in FIG. 2.
Referring to FIGS. 1 and 2, a control block B1 is used for transmitting the control information from the control node 200 to each controlled node 260. This control block B1 periodically receives signals. A given control block is separated from the next control block by plural timeslots SL (four time slots SL1 to SL4 in the example of FIG. 2). Each node transmits the transfer blocks B2 (transfer blocks B2A, B2B and B2C in the example of FIG. 2) within this time slot SL to transfer the data.
Referring to FIG. 3, a portion of the aforementioned control block B1 is used as a use permission signal (transmission permission signal) specifying the permission of use of the time slot SL. This use permission signal is sent by the control node 200 to each controlled node 260. In the example shown in FIGS. 2 and 3, the use permission signal in the control block B1 is first checked and first the controlled node 260A transfers the transfer block B2A to the control node 200. The control node 200 then transfers the transfer block B2B to the totality of the controlled nodes 260. The controlled node 260C transfers the transfer block B2C to the control node 200.
This network is in need of a broad frequency range for realization of a high-speed communication. Also, in order to assure co-existence with the above-mentioned systems, such as remote controller or cordless headphone, this network performs the communication using the subcarrier frequency not lower than 6 MHz, as shown shaded in FIG. 4.
The internal block structure of the control node 200 and the controlled node 260 is shown in FIG. 5.
In this figure, the control node 200 has a transmitter 210 and a receiver 220, while the controlled node 260 also has a transmitter 240 and a receiver 250. The transmitter 210 of the control node 200 has an orthonormal modulation circuit 211 and a light emission circuit 212 while the receiver 220 includes a light reception circuit 221 and an orthonormal demodulation circuit 222. Similarly, the transmitter 240 of the controlled node 260 has an orthonormal modulation circuit 241 and a light emission circuit 242, while the receiver 250 has a light reception circuit 251 and an orthonormal demodulation circuit 252.
The orthonormal modulation circuit 211 of the control node 200 modulates a transmission signal S201 to output a modulated signal S202 composed of frequency components not less than 6 MHz. This modulated signal S202 is inputted to the light emission circuit 212, which then amplitude-modulates the infrared rays based on the modulated signal S202. That is, the light emission circuit 212 includes a light emitting diode LED emitting infrared rays, and drives this light emitting diode LED based on the modulated signal S202. This causes the light emission circuit 212 to output the infrared rays S203 amplitude-modulated based on the modulated signal S202.
In the receiver 250 of the controlled node 260, the infrared rays S203 outputted by the control node 200 are received by the reception circuit 251. This reception circuit 251 includes a photodiode and receives the infrared rays S203 to convert the received rays into electrical signals. The reception circuit 251 also includes e.g., a high-pass filter which cuts the dc components of the electrical signals. An output signal S204 of the reception circuit 251 is inputted to the orthogonal demodulation circuit 252, which then orthogonally demodulates the signal S204 to reproduce the same reception signal S205 as the transmission signal S201.
Meanwhile, the transmitter 240 of the controlled node 260 is substantially of the same structure as the transmitter 210 of the control node 200, while the receiver 220 of the control node 200 is substantially of the same structure as the receiver 250 of the controlled node 260. That is, the orthogonal modulation circuit 241 of the controlled node 260 modulates the transmission signal S211 to output a modulated signal S212 composed of a frequency component not lower than 6 MHz. The light emission circuit 242 amplitude-modulates the infrared light based on the modulated signal S212. The light emission circuit 242 outputs the infrared rays S213 amplitude-modulated based on the modulated signal S212. The receiver 220 of the control node 200 also receives the infrared rays S213 from the controlled node 260 to convert the infrared rays into electrical signals while cutting off the dc components of the electrical signals. An output signal S214 of the reception circuit 221 then is orthogonally demodulated to regenerate the same electrical signals S215 as the transmitted signal S211.
FIG. 6 shows the amplitude (light emission intensity) of the infrared rays S203 modulated on the basis of the modulated signals S202. FIG. 6 shows the control block B1 and the transfer block B2B transmitted by the control node 200.
In the high-speed radio communication, employing infrared rays, as described above, the following problem arises in connection with the communication means for the high-speed radio communication, especially the above-mentioned transmitter.
Since the light emission circuit of the transmitter executes amplitude modulation, as discussed above, the infrared light of a pre-set constant level is perpetually outputted even in the absence of transmission signals, that is when no transmission is being preformed, as may be seen from FIG. 6. That is, since the infrared rays are being outputted in a mode of transmitting a signal only once every 1000 periods, the infrared rays are radiated wastefully for 999 periods, thus increasing the power consumption.
It is therefore an object of the present invention to provide a method and apparatus for optical communication whereby it is possible to reduce the power consumption of the light emission circuit.
In one aspect, the present invention provides an optical communication method used in a communication network in which communication is had between a control node and a plurality of controlled nodes using light rays amplitude-modulated by carrier modulation signals of a first pre-set frequency range. The communication method includes a step of transmitting a transmission permission signal from the control node to each controlled node, and a step of starting or interrupting light emission of the light rays with desired transient characteristics by each controlled node having reference to the transmission permission signal.
In another aspect, the present invention provides an optical communication apparatus used in a communication network doing communication with light rays amplitude-modulated by a carrier modulation signal of a first pre-set frequency range. The communication apparatus includes inputting means for inputting a transmission permission signal transmitted by a node connected to the communication network, and light emission control means for starting or interrupting emission of the light rays at desired transient characteristics by having reference to the transmitted transmission permission signal.
In yet another aspect, the present invention provides an optical communication apparatus used in a communication network in which communication is had between a control node and a plurality of controlled nodes using light rays amplitude-modulated by carrier modulation signals of a first pre-set frequency range. The control node at least includes transmission permission signal transmitting means for transmitting a transmission permission signal to each controlled node. Each controlled node has at least light emission control means for starting or interrupting the emission of the light rays with desired transient characteristics by having reference to the transmission permission signal.
With the optical communication method and apparatus according to the present invention, the light emitting circuit can be operated with low power consumption by the control node sending out a transmission permission signal to each controlled node and by each control node then having reference to the transmission permission signal to start or interrupt the emission of light rays with desired transient characteristics. If infrared rays are used as the light rays, the desired transient characteristics are selected so that no strong spurious will be produced in the subcarrier frequency range of 33 kHz to 6 MHz, whereby it is possible to diminish the light emission time of infrared light rays in the controlled node to decrease the power consumption of the controlled node, by selecting the desired transient characteristics so that no significant spurious will be produced in the subcarrier frequency range of 33 kHz to 6 MHz. This can be achieved as compatibility with existing systems, such as remote controllers or cordless headphone systems, is maintained. On the other hand, in short-distance communication, it is possible to diminish the power consumption of the controlled node and the control node by decreasing the output of the infrared rays and by effecting light emission on/off control instantaneously.